I am reading the book (Pragmatic Programmers Agile Dev. with Rails) which shows how to do unit tests in Rails.

The book showed me the default rails testing suite (a subclass of Test::Unit). Is this the main testing framework that is used in the Rails community? I use RSpec when doing regular ruby, and I would like to be able to use it in Rails too (if it's not too much of a hassle)?

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There are many tools for testing rails and other webapps from many different aspects. But if you are new to testing I highly recommend you start with learning Rails' own testing framework before start using other tools.

Learning, and later mastering, one testing framework makes it easier in the future to understand pros/cons with other framework and make them work in unison.

You could start with testing the following things:

Unit Testing your Models

Functional Tests for Your Controllers

Learning about Fixtures and how to load test data

I have seen many failed testing efforts, but I never saw them fail because they choose the wrong tool/framework. They fail because they don't know how to master the tools they use, and learn enough about the basics about testing.

I would say rspec is the most widely used. It's easy to get it up and running, it's expressive and easy to use. Rails comes with Test::Unit support by default, but I don't see many people using it. And of course there's a myriad of smaller testing frameworks that a small number of people use. But it's mostly rspec.

now that rails 3 is out it is important to note that rails provides test harness agnosticism, allowing you to pick your test solution and enforcing no baked in solution.
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Jed SchneiderSep 9 '10 at 1:24